1. Technical Field
The present invention relates to clad carbon foam tooling articles useful in fabricating composite materials. More particularly, the present invention relates to tooling articles comprising high strength carbon foam blocks clad with durable facesheet materials. Even more particularly, the present invention relates to tooling articles comprising high strength, monolithic carbon foam blocks sealed with cured carbonaceous cements and clad with thermally compatible durable facesheet materials and further relates to processes and materials for forming such articles.
2. Background Art
High strength, light weight carbon materials have been suggested in the prior art as useful for tooling articles. In particular, carbon foams have attracted considerable recent activity because of their low density, coupled with either very high or low thermal conductivity. Unfortunately, carbon foams produced by the prior art processes are not adequate for many high temperature applications such as composite tooling. These foams generally are not monolithic and fail to meet the strength and strength to density requirements for such applications. In addition, open-celled carbon foams with highly interconnected pores have porosities making them unsuitable for such applications. The terms pore and cell are used interchangeably to refer to the small cavities in the foam formed by gaseous displacement in its precursor material.
Cladding of carbon foams has also been suggested in the prior art as an adaptation for improving their usefulness as tooling articles. In their U.S. Pat. No. 6,849,098, Joseph and Rogers describe the carbon foam products having highly interconnected, open-celled pore structures clad with so-called ‘facesheet’ materials. Herein, the term ‘facesheet’ refers to a skin, cladding or outer layer of an article, especially a tooling article, that has a working face or working surface. The facesheet materials applied to carbon foams according to the process described by Joseph and Rogers either completely or partially fill the cell volumes. However, filling cells with facesheet materials will increase the density of such foams and consequently reduce their suitability as a lightweight material for use in composite tooling. Facesheet clad carbon foams created by the Joseph and Rogers process exhibit inherent structural problems, including difficulties in adhering the facesheet materials directly to the carbon foam and internal stressing and cracking of facesheet materials caused by incompatible rates of thermal expansion. The Joseph and Rogers foam products likely do not have the required strength to density ratios needed for tooling or other structural applications. Also, the foams produced in accordance with the Joseph and Rogers techniques are not available in sufficiently large blocks for tooling, thus requiring several blocks to be cemented together. While such blocks may be successfully cemented together, more cemented joints necessarily reduces the structural integrity of the resulting block and can lead to seams in the finished product.
Recently, a carbon foam has been developed and commercialized under the trademark GRAFOAM by UCAR Carbon Company Inc. of Parma, Ohio and described in U.S. Patent Application Publication No. US 2006-0086043 A1 to which the present application claims priority. This novel foam is monolithic and has a controllable cell structure providing a cell structure, strength and strength to density ratios suitable for composite tooling and other applications. Indeed, a combination of characteristics found in GRAFOAM carbon foam, including strength to density ratios higher than demonstrated in the prior art, have been found to be necessary for use of a carbon foam in composite tooling applications.
While GRAFOAM carbon foam has a pore structure which provide low gas permeability in comparison to the open-cell carbon foams available in the market, its carbon foam surface is still porous and requires sealing in order to make the carbon foam especially useful for applications such as tooling. If the foam surface is not adequately sealed, resin can infiltrate the foam block during composite manufacturing processes such as resin infusion and vacuum assisted resin transfer molding. The novel carbon foam has a density of about 0.05 to about 0.8 grams per cubic centimeter (g/cm3), with a compressive strength of at least about 2000 pounds per square inch (psi) (measured by, for instance, ASTM C695) for composite tooling materials; core material is lower density material. The novel carbon foam, when intended for use in high temperature applications such as composite tooling, is formed so as to provide a ratio of strength to density of at least about 7000 psi/(g/cm3).
The novel carbon foam has a cell structure with low interconnectivity. Two distinct pore size distributions greatly improve the suitability of this novel carbon foam for composite tooling applications. One pore size is in the micron range; the other in the tens to hundreds of microns range. However, conventional commercial sealers cannot seal the pores to the extent desired. Low-viscosity commercial sealers simply wick into the foam and cannot seal the surface even after many applications. High-viscosity commercial sealers/adhesives, such as those commercially available as Loctite 9394 and 9396, form sealant skins on the foam surface; such skins shrink and crack during curing, resulting in delamination of the sealant from the foam.
Cement sealants for sealing monolithic graphite or other “solid” carbon block are known in the prior art. Previously disclosed carbonaceous cements include cement paste compositions having finely divided solid carbonaceous particles, such as graphite flour, coke flour, carbon black, pitch coke flour and calcined lampblack flour, that are present at from about 20% to about 85% by weight. Such prior art cements may also include a resin binder system, a solvent and a catalyst. Although effective as cements, there is no disclosure in the prior art of the use of carbonaceous cements to either seal porous carbon foams or bond together blocks of porous carbon foams, especially those having a pore structure uniquely suited for use in applications such as composite tooling.
Recently, a novel sealant comprising two filler fractions having particles of differing size distributions has been developed by UCAR Carbon Company Inc. of Wilmington, Del. and described in above identified U.S. patent application Ser. No. 11/137,111, to which the present application claims priority. The first filler fraction comprises between about 12% to about 50% by weight of the novel sealant material and has a particle size distribution wherein at least 80% of the particles are between about 2 microns and about 500 microns in diameter, with an average diameter of less than about 120 microns. The second filler fraction comprises about 8% to about 35% by weight of the novel sealant material and has an average particle size of between about 0.2 to about 10 microns. The filler particles can be any materials which can be prepared in the desired particle sizes and distributions, including metals and ceramics such as silicon carbide and the filler fractions may be of different materials. The filler particles may be formed of a carbonaceous material in order to more closely match the coefficient of thermal expansion (CTE) of the novel carbon foam.
This novel sealant material can effectively fill the relatively small cells and bimodal cell structure of the novel carbon foam, which has a combination of larger and smaller relatively spherical pores. This novel bimodal pore structure is required if the foam is to be used in applications such as composite tooling. The novel sealant material forms, after curing or low temperature carbonization, a thin layer on a surface of the foam, on the order of about 1000 microns in thickness or less, and is well bonded to the cellular carbon foam surface. Application of this novel sealant effectively converts the carbon foam surface morphology into a monolithic, fine-grain graphite-like surface. The thin layer is well bonded to the carbon foam because the filler particles provide structural stability and also because the filler particles moderate the relatively high coefficient of thermal expansion (CTE) of the resin component so as to provide a sealant coefficient of thermal expansion compatible with the carbon foam coefficient of thermal expansion. The filler particles also help limit the amount of the liquid component in the sealant available to infiltrate into carbon foam. The modified surface morphology of the sealer layer makes it possible to further seal the surface with a typical mold sealer without fillers to provides a vacuum tight surface. The novel sealant applied to the novel carbon foam without use of facesheet materials and the like may be useful for limited composite tooling applications, such as prototyping, where durability of the tooling surface is not of great concern. However, greater tooling surface durability and smoothness is generally required in production applications.
As noted above, cladding of carbon foams has also been suggested in the prior art literature and patents. Joseph and Rogers suggest facesheet materials including Kevlar reinforced carbonaceous foam and laminated E-glas reinforced vinyl esters. Joseph and Rogers also suggest thermal spray applications of coatings of metals to their carbon foam products and further suggest use of aluminum or Inconel nickel-chromium alloy to achieve surface, heat transfer and thermal expansion properties compatible with carbon foam cores. However, it is not known that such a tool has ever been demonstrated.
A preliminary investigation of deposition of sprayed metal upon an open-celled carbon foam surface demonstrated several deficiencies with the prior art applications of thermal or plasma sprayed metal skins. Microscopic examinations of sections of plasma sprayed Invar iron-nickel metal alloy revealed a poorly adhered Invar powder coating that failed to differentially deposit and fill the open cells and pinholes present in the open-celled structure of the carbon foam and, thus, provided a rough pitted surface unsuitable for tooling composite materials.
What is desired therefore, is a composite material tooling article having smooth, durable, tightly adhered skin suitable for use in composite tooling applications; sealants providing a sealed carbon foam surface which enables carbon foams to be employed in high temperature applications such as composite tooling applications; and the carbon foams themselves, especially carbon foams whose pore structure, strength, and strength to density ratio is suitable for use in application such as composite tooling.